1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to radio frequency transmitters used within such systems.
2. Description of Related Art
FIG. 1 is a schematic block diagram of a known super heterodyne radio frequency transmitter architecture. As shown, the transmitter includes a digital baseband (BB) processor, a digital-to-analog converter (DAC), a 1st intermediate frequency mixer, a surface acoustic wave (SAW) filter, a 2nd intermediate frequency mixer, a bandpass filter, a power amplifier (not shown) and an antenna. In operation, the digital baseband processor converts data into digital baseband signals in accordance with a particular encoding protocol as may be prescribed in various standards such as IEEE 802.11, Bluetooth, global system for mobile communications (GSM), et cetera. The digital-to-analog converter converts the baseband digital signal into an analog digital signal. A frequency domain representation of the baseband signal, which includes a plurality of channels, is shown centered at DC.
The 1st intermediate frequency mixer mixes the baseband signal with a 790 MHz local oscillation. As one of average skill in the art will appreciate, the mixer stage may include two mixers for mixing an in-phase component and quadrature component. The resulting intermediate frequency is shown to include a plurality of channels several of which are undesired and one of which is desired. In addition, as a result of the mixing via the 1st mixing stage a local oscillation (LO) leakage is produced and shown at the intermediate frequency (IF). The LO leakage results due to mismatches in the components of the in-phase and quadrature mixers of the first IF mixer as well as mismatches between the in-phase intermediate frequency and quadrature intermediate local oscillation.
The SAW filter removes the undesired channels leaving the desired channel. As shown, the desired channel still includes the local oscillation leakage. The next intermediate frequency stage mixes the desired channel of the IF signal with a 2nd local oscillation (e.g., 4.4 gigahertz) to produce a radio frequency signal. The radio frequency signal, which may have a 5.21 gigahertz frequency, is shown to include the local oscillation leakage centered at RF.
As is known, when an RF signal that includes an LO leakage component is received by a receiver, the LO leakage cannot be filtered out and thus adversely effects the recapturing of data. In some instances, the magnitude of the LO leakage with respect to the magnitude of the signal is such that data cannot be accurately extracted from the RF signal.
Therefore, a need exists for a method and apparatus to reduce LO leakage in a radio frequency transmitter.